Conventional nuclear medicine techniques produce an image depicting the distribution of radioactive compounds within organs of the human body by filtering the high energy photons or gamma rays emitted by those compounds by means of a "collimator." Conventional collimators typically have a thickness of radiation absorbent material punctuated by a large number of parallel, relatively small diameter holes. These holes are of such small diameter compared to the thickness of the collimator that only those photons that enter the holes at the angle of the hole can pass therethrough. The holes of such conventional collimators are typically perpendicular to the plane of their faces. A photon that passes through the collimator produces a flash of light as it strikes a scintillation crystal located behind the collimator. Each such strike is amplified by a photomultiplier to generate a detectable signal. The pattern of these flashes represents the distribution of radioisotope aligned directly in front of each of the holes. The image generated from the processing of a great many of such signals is a two-dimensional projection of a three-dimensional distribution as a conventional X-ray photograph is a two-dimensional projection of a three-dimensional volume of interest. Such two-dimensional images of radioisotope distribution may be achieved by the well-known scintillation camera, also called the Anger camera after its inventor, and described in U.S. Pat. No. 3,011,057.
It is often important to the interpretation of nuclear medicine studies to understand the actual three-dimensional distribution of isotope within the organ being studied. In a conventional nuclear medicine imaging technique, however, one spatial dimension of information is lost each time as image is produced. Though images can be produced from various viewing angles and the resulting pictures considered together, it is often difficult to combine these various views into an understanding of the actual three-dimensional distribution.
Several techniques have been utilized in an attempt to produce three-dimensional "image" of isotope distribution within an organ.
Earlier techniques included moving the patient and/or detector so that radioactivity from other than a single depth is "blurred". One such method is disclosed in U.S. Pat. No. 3,612,865 to Walker. No actual removal of out-of-plane activity occurs, but rather such information is spread out in the image so that any information distant from the plane of interest becomes less pronounced. However, there appears a high background activity in the resulting image. The depth where the blurring is minimized is varied by adjusting the processing circuits. Multiple images may be produced, each emphasizing a different depth.
Other techniques rely on a focusing technique to increase collimator sensitivity at a certain depth. Several techniques for achieving depth discrimination using rectilinear scanning including inclining the detectors is disclosed in Kuhl, et al., "Image separation Radioisotope Scanning" 80 Radiology (1963) 653.
More recent work has begun to utilize various reconstruction techniques to separate data from multiple views into separate planes, each representing the isotope distribution at a specific depth from the front of the detector.
Several reconstruction methods to generate a three-dimensional density distribution are discussed in Gilbert, P., "Iterative Methods for the Three-dimensional Reconstruction of an Object from Projections," 36 J. Theor. Biol. (1972) 105. Gilbert discusses and compares several iterative reconstruction methods including the Algebraic Reconstruction Technique (ART) and the Simultaneous Iterative Reconstruction Technique (SIRT).
Various techniques have been used to produce multiple views and to optimize the information obtained from them. These include various types of coded apertures, freznel zone plates, and multiple overlapping pinholes. All of these techniques have performed well with limited source distributions, but background activity and extended sources have limited the quality of the images.
A recent development is the seven pinhole, non-overlapping images technique described by Vogel et al., 19 Journal of Nuclear Medicine (1978) 648. Seven independent projections are produced simultaneously, through seven pinholes, each viewing a common volume directly in front of the collimator. The seven images are processed by a reconstruction technique to yield multiple planar images, each representing a slice through the common volume. This technique has shown improved detection sensitivity in comparison with conventional collimators.